Overheating Honda accord

My 1994 honda accord lx has a new radiator and water pump 6 months ago. Went to ez lube last week. I checked the water level, it was fine, then after driving 15

minutes on the fwy, she began to overheat. But when I slowed down, the water gauge lowered, then accelerating made it hotter until I exited the fwy and let it sit awhile. Started her up again and again it slowly got hotter.Made it home(15 miles),

let it cool and checked the water again, but just the reserve was low. There was no puddle under the engine. Took her to the place that replaced the radiator and water pump and they checked for leaks, but none were found. The mechanic said maybe the water evaporated out the exhaust and he could put some kind of sealant in for $100 that had a 50% chance of working.Couldn’t I buy that same product at pep boys for $10 and do it myself? But my question to you is "What could be the matter, and should I just move on and get

another car?

Thanks in advance for your response…

P.S. Should I just ask the Tappit Bros. on Car Talk?

I’m leaning toward a thermostat replacement. The thermostat controls coolant flow through the radiator. If it doesn’t let coolant flow, the harder the engine works, the hotter it gets.

I second the thermostat. I change mine every 4-5 years.
Yours if original is at a good age to fail.

Have a real mechanic/shop check for a blown head gasket…(after replacing the thermostat). If there is exhaust in the coolant, park it down along the back fence next to the others and bring the keys and plates back to the office…

Thank u hellokit, circuitsmith and Caddyman for your advice, by way of this most trustworthy of forums, not to mention the only place I would ever trust in the advice of complete strangers.
That being said, how much does a thermostat cost and the labour? Can I buy one from the junk yard people downtown L.A?
Can I do it myself?
The old phrase"money is no object" does not apply in this case.
Thanks ahead of time.

I would put in a thermostat from Honda, but I’m picky. About $10-15 with gasket.
Aftermarket is less of course.
You can DIY with a 10mm (or 12mm?) socket wrench. 30 minute job. Get some antiseize grease for the bolt threads.